With continued regulatory pressure there is a growing need to identify more environmentally sustainable replacements for refrigerants, heat transfer fluids, foam blowing agents, solvents, and aerosols with lower ozone depleting and global warming potentials. Chlorofluorocarbon (CFC) and hydrochlorofluorocarbons (HCFC), widely used for these applications, are ozone depleting substances and are being phased out in accordance with guidelines of the Montreal Protocol. Hydrofluorocarbons (HFC) are a leading replacement for CFCs and HCFCs in many applications; though they are deemed “friendly” to the ozone layer they still generally possess high global warming potentials. One new class of compounds that has been identified to replace ozone depleting or high global warming substances are halogenated olefins, such as hydrofluoroolefins (HFO) and hydrochlorofluoroolefins (HCFO). In the present invention, it was discovered that chloro-trifluoropropenes are particularly useful refrigerants liquid chiller systems, particularly in negative-pressure chiller systems, such as for the replacement of R-11 and R-123.
With continued regulatory pressure there is a growing need to identify more environmentally sustainable replacements for refrigerants, heat transfer fluids, foam blowing agents, solvents, and aerosols with lower ozone depleting and global warming potentials. Chlorofluorocarbon (CFC) and hydrochlorofluorocarbons (HCFC), widely used for these applications, are ozone depleting substances and are being phased out in accordance with guidelines of the Montreal Protocol. Hydrofluorocarbons (HFC) are a leading replacement for CFCs and HCFCs in many applications; though they are deemed “friendly” to the ozone layer they still generally possess high global warming potentials. One new class of compounds that has been identified to replace ozone depleting or high global warming substances are halogenated olefins, such as hydrofluoroolefins (HFO) and hydrochlorofluoroolefins (HCFO). The HFOs and HCFOs provide the low global warming potential and zero or near zero ozone depletion properties desired.
Chillers are refrigeration machines that cool water, other heat transfer fluids, or process fluids by a vapor-compression (modified reverse-Rankine), absorption, or other thermodynamic cycle. Their most common use is in central systems to air condition large office, commercial, medical, entertainment, residential high-rise, and similar buildings or clusters of buildings. Both large central and interconnected plants, generally with multiple chillers in each, are common for shopping centers, university, medical, and office campuses; military installations; and district cooling systems. The chilled water (or less commonly a brine or other heat-transfer fluid) is piped through the building or buildings to other devices, such as zoned air handlers, that use the cooled water or brine to air condition (cool and dehumidify) occupied or controlled spaces. By their nature, both efficiency and reliability are critical attributes of chillers. Chillers typically range in thermal capacity from approximately 10 kW (3 ton) to exceeding 30 MW (8,500 ton), with a more common range of 300 kW (85 ton) to 14 MW (4,000 ton). Larger systems typically employ multiple chillers, with some installations exceeding 300 MW (85,000 ton) of cooling. Liquid-chilling systems cool water, brine, or other secondary coolant for air conditioning or refrigeration. The system may be either factory-assembled and wired or shipped in sections for erection in the field. The most frequent application is water chilling for air conditioning, although brine cooling for low temperature refrigeration and chilling fluids in industrial processes are also common.
The basic components of a vapor-compression, liquid-chilling system include a compressor, liquid cooler (evaporator), condenser, compressor drive, liquid-refrigerant expansion or flow control device, and control center; it may also include a receiver, economizer, expansion turbine, and/or subcooler. In addition, auxiliary components may be used, such as a lubricant cooler, lubricant separator, lubricant-return device, purge unit, lubricant pump, refrigerant transfer unit, refrigerant vents, and/or additional control valves.
Liquid (usually water) enters the cooler, where it is chilled by liquid refrigerant evaporating at a lower temperature. The refrigerant vaporizes and is drawn into the compressor, which increases the pressure and temperature of the gas so that it may be condensed at the higher temperature in the condenser. The condenser cooling medium is warmed in the process. The condensed liquid refrigerant then flows back to the evaporator through an expansion device. Some of the liquid refrigerant changes to vapor (flashes) as pressure drops between the condenser and the evaporator. Flashing cools the liquid to the saturated temperature at evaporator pressure. It produces no refrigeration in the cooler. The following modifications (sometimes combined for maximum effect) reduce flash gas and increase the net refrigeration per unit of power consumption.
Subcooling. Condensed refrigerant may be subcooled below its saturated condensing temperature in either the subcooler section of a water-cooled condenser or a separate heat exchanger. Subcooling reduces flashing and increases the refrigeration effect in the chiller.
Economizing. This process can occur either in a direct expansion (DX), an expansion turbine, or a flash system. In a DX system, the main liquid refrigerant is usually cooled in the shell of a shell-and-tube heat exchanger, at condensing pressure, from the saturated condensing temperature to within several degrees of the intermediate saturated temperature. Before cooling, a small portion of the liquid flashes and evaporates in the tube side of the heat exchanger to cool the main liquid flow. Although subcooled, the liquid is still at the condensing pressure.
An expansion turbine extracts rotating energy as a portion of the refrigerant vaporizes. As in the DX system, the remaining liquid is supplied to the cooler at intermediate pressure. In a flash system, the entire liquid flow is expanded to intermediate pressure in a vessel that supplies liquid to the cooler at saturated intermediate pressure; however, the liquid is at intermediate pressure.
Flash gas enters the compressor either at an intermediate stage of a multistage centrifugal compressor, at the intermediate stage of an integral two-stage reciprocating compressor, at an intermediate pressure port of a screw compressor, or at the inlet of a high-pressure stage on a multistage reciprocating or screw compressor.
Liquid Injection. Condensed liquid is throttled to the intermediate pressure and injected into the second-stage suction of the compressor to prevent excessively high discharge temperatures and, in the case of centrifugal machines, to reduce noise. For screw compressors, condensed liquid is injected into a port fixed at slightly below discharge pressure to provide lubricant cooling.